RFR: 8328481: Implement Module Imports [v2]

[Jan Lahoda]

> This is an implementation of JEP JDK-8315129: Module Import Declarations (Preview). Please see the JEP for details:
> https://bugs.openjdk.org/browse/JDK-8315129
> 
> It is mostly straightforward - the module imports are parsed, and then expanded to import-on-demand in `TypeEnter`.
> There is a few notable aspects, however:
> - the AST node for import (`JCImport`) is holding the imported element as a field access, because so far, the imported element always had to have a '.' (even for import-on-demand). But for module imports, it is permissible to import from a module whose name does not have a dot (`import module m;`). The use of field access for ordinary import seems very useful, so I preferred to keep that, and created a new internal-only AST node for module imports. There is still only one public API AST node/interface, so this is purely an implementation choice.
> - JShell now supports module imports as well; and the default, implicit, script is changed to use it to import all of `java.base` if preview is enabled. It is expected that the default would be changed if/when the module imports feature is finalized.

Jan Lahoda has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 16 additional commits since the last revision:

 - Merge branch 'master' into module-imports
 - Merge branch 'master' into module-imports
 - Fixing test.
 - Fixing disambiguation of module imports.
 - Fixing file name computation.
 - Merge branch 'master' into module-imports
 - Cleanup.
 - Fixing CheckExamples.
 - Adding support for import modules to JShell.
 - More correct handling of requires transitive.
 - ... and 6 more: https://git.openjdk.org/jdk/compare/c0c0131b...3f2deab9

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18614/files
  - new: https://git.openjdk.org/jdk/pull/18614/files/9cab0ae8..3f2deab9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18614&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18614&range=00-01

  Stats: 318 lines in 18 files changed: 46 ins; 198 del; 74 mod
  Patch: https://git.openjdk.org/jdk/pull/18614.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18614/head:pull/18614

PR: https://git.openjdk.org/jdk/pull/18614